Rigid-gas-permeable contact lenses, i.e., "RGP" lenses have been available for many years. Many lens designs have been suggested for RGP lenses. Generally, the lenses include one or more concave curves on the posterior (back or ocular) surface of the lens and one or more convex curves on the anterior (front) surface. The curves may be spherical or aspherical. See, for example, U.S. Pat. No. 2,544,246 to Butterfield for a spherically curved lens and U.S. Pat. No. 4,883,350 to Muckenhirn for an aspherically curved lens design.
Although the vast majority of lenses currently prescribed are single vision lenses, there is an increasing interest in bifocal or multifocal contact lenses for persons with presbyopia ("presbyopes"), due to an aging population. This will result in increasing numbers of presbyopes, since everyone becomes presbyopic with age. Presbyopia is a gradual lessening of the power of accommodation due to a physiological change that first becomes noticeable about the age of 40 years. Accommodation refers to adjustment by the eye for seeing at different distances, which accommodation is accomplished by changing the shape of the crystalline lens within the eye through the action of the ciliary muscle which are attached to opposite longitudinal ends of the crystalline lens. Accommodative ability is believed to decrease with age because of the loss of strength of the ciliary muscle and/or hardening of the lens. The primary feature of presbyopia is the inability to do close work. Myopic (near-sighted) persons who develop presbyopia will, therefore, eventually need both near-vision and distance-vision correction. Multifocal lenses correct for both near- and distance-vision for presbyopes. Such lenses are also suited for patients with aphakia, accommodative convergence defects, and the like.
Existing multifocal lenses can be broadly categorized as having either simultaneous or alternating designs. An alternating design requires lens translation to present the eye alternatively with distance- and near-lens segments of the lens. A simultaneous design does not require lens translation but rather involves focusing both the distance and the near images at the same time. For example, one concentric simultaneous design has a circular near segment surrounded by an annular distance segment (center-near) or the reverse (center-distance). U.S. Pat. No. 5,798,817 to de Carle discloses a plurality of such concentric near-vision and distance-vision zones.
Aspheric multifocal lenses are a type of concentric simultaneous lens having one or more aspheric curves from the center to the edge of the lens. An aspheric curve may be on the posterior (ocular) side of the lens or on the anterior side of the lens, or both. A commercially successful, simultaneous, concentric multifocal lens with a posterior aspheric design is the Boston MultiVision.RTM. lens, which is based on the design disclosed in U.S. Pat. No. 5,436,678 to Carroll. This lens is designed to reduce de-centering of the lens on the eye and to minimize the variability of performance caused by varying pupil sizes. The lens is described as comprising a central zone, a marginal zone, and a peripheral zone, wherein the axial lift of the central and marginal zones increase towards the edge of the lens and the axial lift of the peripheral zone decreases toward the edge of the lens.
Clinical experience has proven the Boston MultiVision.RTM. lens design to be especially advantageous, in terms of performance and comfort, for presbyopic contact-lens wearers, especially for relatively early presbyopes. For more advanced presbyopes, however, it would be desirable to have a lens that is specifically designed for providing greater refractive correction than typically provided by the Boston MultiVision.RTM. lens. In particular, it would be desirable to increase the near-vision correction of the latter lens without sacrificing the performance of the lens, including its already excellent visual acuity, distance correction and comfort. Finally, irrespective of use with the Boston MultiVision.RTM. lens, it would be desirable to provide an improved design for the anterior surface of multifocal lens in general, either as a stand-alone for vision correction or to provide additional power to the posterior surface of the lens.